This invention relates to ion exchange resins, and in particular to anion exchange resins having a low chlorine content.
Anion exchange resins are employed in numerous water treatment applications in order to purify streams which are in contact with metals, especially ferrous metals. Unfortunately, such resins contain chlorine in the form of chloride ions and organic chloride which can leach into the treated fluid to cause corrosion of said metals.
Strong anion exchange resins in the normal chloride form that have been treated to convert them to the hydroxide form will tend to show an increase in active ionic chloride immediately after conversion and subsequently during storage. The active ionic chloride used herein is the chloride in ionic form which is leachable from the resin, as distinguished from covalent chlorine which is strongly bound and releasable usually only upon destruction of the resin.
It is thought the source of the increased active chloride is unreacted chlorine sites in the polymer network resulting from incomplete functionalization of the resin or chlorine introduced to the inert copolymer matrix which thereafter converts from covalent to ionic form upon conversion of the resin to the hydroxide form. It is further believed that the functional group of the ion exchange resin in hydroxide form is capable of causing migration of chlorine within the molecule.
Chloride ion can be displaced from a resin by treating the resin with (1) solutions containing ions for which the resin has a selectivity in preference approaching, equaling or exceeding chloride; (2) highly concentrated solutions containing ions for which the resin has low to high selectivity; and (3) fluids containing complexes for which the resin has high affinity. Additionally, resins having high chloride content can lose chloride to relatively pure solvents or solvent mixtures under conditions of high temperature, pressure, and the like.
Unfortunately, conversion of the normal chloride form of an anion exchange resin to the hydroxide form by the prior art methods has not achieved the goal of completely removing "active" chloride ions. Typical low chloride content anion exchange resins contain about 2 to about 10 equivalent percent chloride, based on total exchangeable ions. Such resins are clearly limited in the amount of their functional capacity in the hydroxide form.
More recently, as described in U.S. Pat. No. 4,025,467, there is disclosed a process for preparing strong-base anion exchange resins containing less than 0.5 percent active chloride. Such a process involves the solvolytic displacement of non-ionic chloride ions. Such a process involves treating the resin in an aqueous medium under harshly basic conditions and relatively high temperatures. Such harsh treatments are undesirable because they are time-consuming, uneconomical, and provide ion exchange resins which are more susceptible to breakage and have lower exchange capacity than is desirable.
In view of the deficiencies of the prior art, it would be highly desirable to provide a process for preparing an ion exchange resin having a very low chloride content, which resin can be prepared in the absence of undesirable harsh conditions.